The present invention relates to tire slip preventing apparatus to be fitted to tires for the purpose of running on a snowy or icy road and, more particularly, to fastening devices for tire slip preventing apparatus which are capable of automatically giving uniform and sufficient fastening forces to anti-slip bodies fitted to the right and left tires of a vehicle.
In recent years, the spread of tire slip preventing apparatus which use meshed strips as anti-slip bodies has proceeded. The meshed strips are formed of an elastic material such as rubber or synthetic resin instead of conventional metal chains. As a fastening device for use in fitting such a tire slip preventing apparatus to a tire, a so-called rope buckle type of fastening device which is easy to fit is becoming popular instead of a conventional rubber ring.
This kind of fastening device, such as that shown in FIGS. 4 and 5, is proposed in Japanese Patent Laid-Open No. 106928/1994. As shown in FIGS. 4 and 5, in a fastening device 10, a fastening band 20 in which a non-extensible rope 21 having flexibility and a rubber band 25 shorter than the non-extensible rope 21 are endlessly connected to each other is formed into a double loop shape, and folded portions 22 and 23 consisting of the non-extensible rope 21 are formed at the opposite ends of the fastening band 20. The folded portion 22 is engaged with an engagement part 31 of a fastening buckle 30, whereas the folded portion 23 is passed over a cleat 32 which is supported on the fastening buckle 30 for pivotal motion on a shaft 35. The cleat 32 faces a stopper which is an inner wall 38 of the body of the fastening buckle 30, across a rope path, and constitutes a non-reversible self-lock mechanism which permits travel of the non-extensible rope 21 in only one direction in the folded portion 23 of the fastening band 20 as will be described later.
FIG. 6 is a front view of the portion of a tire in which the fastening buckle 30 is located, and shows the state in which a tire slip preventing apparatus is fitted by the use of the above-described fastening device 10. An anti-slip body 40 is wrapped around the outer circumference of a tire 50 in such a manner that joining portions 41 and 43 at both lengthwise ends of the anti-slip body 40 (as viewed in the circumferential direction of the tire 50) are connected to each other and the fastening band 20 of the fastening device 10 is hooked on and guided by a plurality of hook metals 45 which are attached at predetermined intervals around a side edge of the anti-slip body 40 (a widthwise side edge of the tire 50). On the vehicle side of the tire 50, a fastening rope is attached at predetermined intervals around the other side edge of the anti-slip body 40, and both ends of the fastening rope are connected to each other at the joining portion 41 and 43.
If the fastening device 10 is to be fitted, the lengthwise joining portions 41 and 43 of the anti-slip body 40 are first connected to each other on the vehicle-body side (reverse side) of the tire 50. Then, after U-shaped hooks 36 and 37 of the fastening buckle 30 are respectively fitted through engagement holes 42 and 44 formed in the respective joining portions 41 and 43 of the anti-slip body 40, both the inner and outer loops of the fastening band 20 are bundled by pulling and are sequentially passed over the plurality of hook metals 45.
After the fastening device 10 has been passed over the side edge of the anti-slip body 40 in the above-described manner, if an outer circumferential loop L.sub.o of the folded portion 23 of the fastening band 20 is pulled in a diameter enlarging direction A, as shown in FIG. 5, the cleat 32 rotates about a shaft 35 toward the right and causes the non-extensible rope 21 to travel around the cleat 32 while holding the shown lock-release position, thereby reducing the diameter of the inner circumferential loop L.sub.i. On the other hand, if the tensile force applied to an outer circumferential loop L.sub.o is released when a tension acting in the opposite direction to the diameter enlarging direction A increases by the reduction in the diameter of the inner circumferential loop L.sub.i, the cleat 32 rotates about the shaft 35 toward the left by the tension of the inner circumferential loop L.sub.i, the top end of the cleat 32 clamps and presses the non-extensible rope 21 against the inner wall 38 of the body of the fastening buckle 30, thereby self-locking the travel of the non-extensible rope 21.
However, the tire slip preventing apparatus using the above-described fastening device 10 has the disadvantage that if such tire slip preventing apparatus are fitted to both right and left tires of a vehicle, the right and left fastening forces do not become uniform during running of the vehicle even if the tire slip preventing apparatus are fastened with the same fastening force. Another problem is that if an initial fastening force is insufficient, the insufficiently fastened state remains unimproved.
As shown in FIG. 3(a), if the rotating direction of the tire 50 is a direction R, when the joining portions 41 and 43 come into contact with a road surface, the fastening buckle 30 which is connected to the joining portions 41 and 43 is subjected to a moment M.sub.L in the opposite direction to the rotating direction R of the tire 50. If the fastening buckle 30 is subjected to the moment M.sub.L, a tensile force A which tends to enlarge the diameter of the outer circumferential loop L.sub.o acts on the fastening band 20 passed over the cleat 32. This tensile force A reduces the diameter of the inner circumferential loop L.sub.i of the fastening band 20 and automatically increases its fastening force. Therefore, the fastening band 20 changes into the state of giving an appropriate fastening force to the anti-slip body 40.
On the other hand, as shown in FIG. 3(b), if the fastening buckle 30 is subjected to the moment M.sub.R in the opposite direction to the rotating direction L of the tire 50, the inner wall 38 acts on the fastening band 20 passed over the cleat 32 so as to increasingly enhance the clamping force applied to the cleat 32, unlike the case shown in FIG. 3(a). Accordingly, the tensile force given to the fastening band 20 by the first fastening operation does not change at all. This leads to the problem that if an insufficient fastening force is given to the fastening band 20, the difference in fastening force between opposite tires becomes larger and larger, and an insufficient fastening force remains unimproved.